The present work involves three major projects within the broad problem area defined in the project title: 1) Comparison of the hamster with the rat with respect to sex differences, especially in sensitivity to shock, DRL behavior, and active avoidance behavior. 2) Examination of the possibility that sex differences in sensitivity to shock in the rat are related to sex differences in brain serotonin levels and 3) investigation of the bases of sex differences in the effects of caudate lesions on open field and active avoidance behavior. Specific studies will examine the influence of gonadal hormones and test the possibility that sex differences in response to damage to the dopaminergic nigrostriatal fiber system underly the differential effects of the lesions. BIBLIOGRAPHIC REFERENCES: Beatty, W.W., Studelska, D.R. and Gerth, J.M. Some aspects of the development of sex differences in DRL behavior. Bull. Psychon. Soc., 1975, 6, 622-624. Beatty, W.W., Bierley, C.M. and Gerth, J.M. Effects of neonatal gonadectomy on DRL behavior. Bull. Psychon. Soc., 1975, 6, 615.